Table 5.
Calculated energy budget expressed as the ratio of the amount in bundle‐sheath (BS) cells to the total in the two cell types, for two NADP‐ME species (I, II) and two NAD‐ME species (III, IV) as reported by Ghannoum et al. (2005), either when no photorespiration nor alternative electron and ATP sinks are assumed (before/) or when photorespiration and alternative electron and ATP sinks are considered (after/)
| I | II | III | IV | |
|---|---|---|---|---|
| Fraction of energy production in BS cellsa | ||||
| NADPH | 0.01/0.01 | 0.05/0.06 | 0.41/0.41 | 0.40/0.40 |
| ATP | 0.35/0.36 | 0.39/0.39 | 0.50/0.50 | 0.49/0.49 |
| Calculated required fraction γb | ||||
| NADPH | 0.59/0.56 | 0.63/0.61 | 0.41/0.43 | 0.40/0.42 |
| ATP | 0.43/0.38 | 0.54/0.46 | 0.83/0.76 | 0.80/0.74 |
| Required η in the ‘aspartate–malate’ mechanismc | 0.73/0.68 | 0.84/0.73 | d | d |
| Required η in the ‘PEP‐CK’ mechanismc | 1.70/1.88e | 1.39/1.56e | 0.78/0.75 | 0.80/0.78 |
| Required values in the triple decarboxylation pathwayf | ||||
| η1 | 0.67/0.75 | 0.57/0.65 | ||
| η2 | 0.27/0.30 | 0.23/0.26 | ||
The most likely estimates for the fraction of 3‐PGA reduction in BS cells and for the fraction of a mixed decarboxylation (see Discussion) are shown in bold type.
This fraction, as an output (see Table 2) of our analytical model, depends on C4 subtypes, among which cell‐type‐specific ATP requirement differs. So, in principle, this fraction calculated by the model varies mathematically once a mixed decarboxylation pathway is involved. However, the modelled results for this fraction only varied after two decimal digits, and this minor change is not given in this Table.
γ, required fraction of NADPH or ATP consumption during the reductive phase of the Calvin cycle that takes place in the BS cells, if a secondary decarboxylation pathway is not engaged.
η, required fraction of OAA that follows the primary decarboxylation pathway if a secondary decarboxylation pathway (either the ‘aspartate–malate’ or the ‘PEP‐CK’ pathway) is also engaged.
No η values were calculated here in Cases III and IV (i.e. two NAD‐ME species) because the cell‐type‐specific NADPH and ATP requirements for the supplementary ‘aspartate–malate’ mechanism are the same as those for the NAD‐ME species (Table 4).
The calculated η values for Cases I and II here are above 1.0, which is physiologically impossible; they are presented here merely to show the results of mathematical calculation (see Discussion).
The calculated values of η1 and η2 (η1 = fraction of C4 acids following the primary NADP‐ME route; η2, the fraction following the ‘aspartate–malate’ route) in a triple decarboxylation pathway in Cases I and II of two NADP‐ME species (see Discussion).